Use the left and right triangles. (the two smaller ones -- ACD and BCD) By similar triangles ACD is similar to BCD on the left AD/CD is one ratio On the right CD/DB is the other. These two ratios are equal because they are corresponding parts.
AD / CD = CD / DB AD = 3 DB = 12 CD = ?? Substitute. 3/CD = CD/12 Cross multiply 36 = CD^2 sqrt(36) = sqrt(CD^2) CD = 6 <<<<==== Answer. A <<<<====Answer.